'Being a touring band, we see all these artists around us who are so talented, and they're just dying in the trenches, and nobody gives a shit," frowns Jim James. "Then we see the people who're selling millions of records and flying in private jets and everything, and they're just like marketing projects, scientific experiments created to look and sound like the real thing. It's such a fuckin' head-trip."

It's an uncommonly grouchy pronouncement from a man more typically given to bonhomie, but then the music industry's machinations are enough to sour anyone's mood. James's band, My Morning Jacket, are by no means "dying in the trenches" – their sixth album, Circuital, went top five when it was released in America this summer, their highest chart-placing yet, and a few hours after our interview they play a sold-out Somerset House in London. But the hirsute, softly spoken Kentuckian knows they're no marketing department's "science project", either.

My Morning Jacket formed in the most organic fashion imaginable: friends who'd known each other since childhood – guitarist Johnny Quaid is James's cousin – making music in a shack located on Quaid's grandparents' farm in Shelbyville, Kentucky. Their fame grew by word of mouth. Early albums such as The Tennessee Fire, At Dawn and It Still Moves – recorded on the farm, James's vocals bathed in the glorious natural reverb of the disused grain silo in which he recorded them – seduced critics and fans with their ethereal psychedelia and exultant rock crescendos.

These early albums arguably coined the cosmic Americana currently purveyed by Fleet Foxes and a generation of similarly starry-eyed indie rockers. "I don't feel slighted when I see bands doing what we were doing back then getting so much more attention than we did then," James says. "Fleet Foxes are a really talented band, they make beautiful music. But it's also fuckin' weird. Like, did we just not do it long enough? If we'd made one more record of harmonies and folk shit, would the press have been, like, 'These guys are fuckin' gods!'?"

As it is, My Morning Jacket's next album, 2005's Z, largely abandoned the "harmonies and folk shit", along with the studio on the farm and its magical grain silo. The year before had been one of upheaval for the group, as Quaid and keyboardist Danny Cash left, wanting more time with their families, and were replaced by guitarist Carl Broeml and keyboard player Bo Koster. A restless James put down the guitar, writing much of the new album on keyboards and a bizarre 80s synthesiser called the Omnichord. Recorded in "a crazy mansion" with Stone Roses producer John Leckie, Z dialled back the classic rock aura of the Jacket's earlier albums, drawing magic from soulful reveries, spectral fairground themes, and even that eternal critical bete noir, rock-reggae.

Though Z got good reviews and sold well, promoting the album almost left James dead in the trenches; shortly after a long American tour in 2005, he was rushed to hospital with a suspected heart attack. Suffering from undiagnosed "walking pneumonia" for much of the tour, James had developed pericarditis, the lining of his heart inflaming to protect it from the infection, almost killing him.

"It was hugely painful, horrifically scary," James says. "I'd just run myself ragged from touring. I almost died, and I was forced to rest up for several months." The trauma would inform the next My Morning Jacket album, which James wrote while recuperating. Redoubling the bold creative leaps of the previous album, 2008's Evil Urges would prove their most successful yet; the resulting tour peaked with a legendary headlining set at that summer's Bonnaroo festival in Tennessee, where MMJ played four hours of their own songs and covers.

With its breathless switching from genre to genre, its overload of ideas, Evil Urges sounds like a memento mori: if the reaper's waiting for you, there's no reason not to take any creative risk you fancy. "For me, Evil Urges was like a video game," says James. "If you play Super Mario Brothers, there's a level where it's like a snowscape, and then there's a level where it's a desert, and a level that's like a jungle. I wanted to make all these different levels for the album, so you were like: 'Oh fuck, what's going on here?' You walk in the next room and it's kind of mellow, and then you walk into the next room and you're like, what the fuck's going on?"

The album's most outre moments wrong-footed some of the more conservative elements of the group's long-term fanbase, who wondered why the boy who'd earnestly penned all those haunting campfire ballads was now indulging in lurid Prince-gone-metal vamps. But James is unrepentant. "It confused some people," he admits. "Highly Suspicious [the group's funk-rock experiment] wasn't ironic, I wasn't making fun of it, but I thought it was fun and hilarious. I guess people have a hard time dealing with humour in music. But sometimes life is depressing, and sometimes life is fun, is about just laughing with your friends, and I wanted to express that, as well as the darker stuff."

The group's expert balance of poignancy and surreal playfulness won them a fan in film-maker Todd Haynes, who cast James in his impressionistic Dylan biopic I'm Not There, dressed in a Sgt Pepper jacket and whiteface and performing I'm Goin' to Acapulco with Calexico at a funeral in the old west, as Richard Gere and a giraffe watch. "There's always something mysterious and multiple about where My Morning Jacket is coming from," Haynes has said; earlier this year, he filmed a My Morning Jacket concert movie, Unstaged, complete with backstage vignettes involving robot imposters of the band, and a guest appearance from radical soulstress Erykah Badu. "Todd gets how weird our music is," says James, "and he's a fucking glorious weirdo himself."

James's ability to evoke the sadness and joy of life, innocence and pain, is much of his music's charm, and he says he was influenced from an early age by The Muppets, who could similarly flit from larks to poignancy with affecting grace. James is enough of a Jim Henson lover that when My Morning Jacket were asked to contribute to a forthcoming Muppets tribute album, they chose to cover obscure Muppet gem Our World rather than, say, Rainbow Connection, a more obvious choice for James' plaintive tones.

"It's from Emmet Otter's Jug-Band Christmas, which is a classic Christmas movie," he grins. "We were gonna make an album as the Electric Mayhem, the rock band from the original Muppet Show. And the plan was we'd go on tour and play behind a curtain, like the Gorillaz, and have puppets or holograms or whatever 'perform' on stage. But it was a Disney project, and Disney's a huge corporation; when one dude gets fired, the next dude who takes over doesn't give a shit about your Muppets project."

Two songs James wrote for the aborted Electric Mayhem project – Wonderful and Outta My System – made it on to Circuital. The chorus of the title song sings of finding oneself "right back in the same place where we started out", and so it was with My Morning Jacket, who returned to their home town of Louisville to record for the first time since leaving the farm. After the luxurious New York studio Avatar, where they recorded Evil Urges and where their every whim was catered for, the group once again found themselves building their own studio, this time transforming an abandoned gymnasium with the aid of Decemberists producer Tucker Martine and several rolls of carpeting they bought from a nearby Salvation Army store.

"It was like Boy Scout camp: just the seven of us in this massive room, no staff or nothin'," James says, of the Circuital sessions. A man who clearly wouldn't want a private jet even if he could afford one, he adds, grinning, "If the tape machine was fucked, we couldn't call in an engineer, we had to work out how to fix it ourselves. And I think that sort of approach gives us more of a sense of triumph about it all."